Coal generally has a sulfur content of 1 to 4 mass percent although it can be as high as 10% for coals mined from seams with large pyrite deposits. Sulfur occurs in two basic forms in coal, either organic or inorganic. The ratio of inorganic to organic sulfur ranges from 4:1 to 1:3 with 2:1 being most common.
The major form of sulfur found in coal is organic. In contrast to inorganic sulfur, organic sulfur is homogeneously distributed throughout the carbonaceous matrix of the coal. Two basic methods exist for the removal of sulfur from coal prior to combustion, namely physical and chemical. Physical desulfurization seeks to separate the sulfur from the coal without altering the sulfur or breaking chemical bonds in the process. The fact that no chemical bonds are broken implies that only the inorganic sulfur forms which, unlike the organic sulfur, are not intimately bound in the carbonaceous matrix can be physically removed. Several methods of physical cleaning of sulfur from coal are currently in use. Although most physical beneficiation methods are relatively inexpensive, only inorganic sulfur forms are removed from the coal. Since current regulations regarding SO.sub.2 emissions require 90% sulfur removal for most coals, removal of organic sulfur is necessary as well. Because organic sulfur is covalently bonded to carbon in the coal, processes energetically capable of breaking carbon-sulfur bonds are required.
Reductive desulfurization results in the removal of inorganic and organic sulfur as H.sub.2 S. The major disadvantages of this process include the high cost of maintaining the high temperature, about 450.degree. C., and pressure, about 2500 psi, required for reaction and the high cost of hydrogen.
On the other hand, oxidative desulfurization schemes seek to remove as much sulfur as possible while minimizing alterations in the rest of the coal structure. This approach is best accomplished by using cheap oxidants under ambient conditions. The selectivity of the oxidant for sulfur with reference to the rest of the coal is of crucial importance to minimize the loss in heating value of the coal. Several oxidative desulfurization schemes have been developed. Most of these attain high inorganic sulfur removal without significant losses in heating value. However, significant organic sulfur removal is usually accompanied by large amounts of oxidation of the carbonaceous material.
In U.S. Pat. No. 3,909,211, the reaction of NO.sub.2 and coal in the gas phase has been investigated. The treated coal is washed in water to remove ferric sulfate. Further treatment with aqueous caustic then reduces organic sulfur. For example, this patent teaches the use of a coal containing 3.6% pyrite sulfur and 0.2% organic sulfur, the coal being treated for three hours in a batch reactor at 100.degree. C. with a gaseous mixture containing 5 to 10% NO.sub.2. The treated coal is then washed with water followed by a wash with 10% NaOH (aq), leaving a coal with only 0.5% total sulfur content. However, the patent is silent as to the mass and heating value losses of the coal so that no conclusions can be drawn as to the effectiveness of this method in removing sulfur without affecting the coal structure itself.
Because processes which have heretofore been used to remove organic sulfur from fossil fuels have been substantially ineffective without significantly affecting the fuel structure, a need has arisen for an improved process for accomplishing this end, and it is the fulfillment of this need to which the present invention is directed.